One known system for producing coextrusions involves the injection of at least one extrudate onto or into a moving stream of another extrudate both of which then pass to a suitable sheet forming extrusion die. U.S. Pat. No. 3,959,431 provides such an apparatus, employing a rod extruder, an adaptor with a lateral feed tube and, a sheet extrusion die. The adaptor carries an introducing means in its center forming an apex. An extruded rod of material is fed through the adaptor and is split internally as it passes around the apex of the introducing means. It is at this point that a second extrudate, fed through the introducing means, is injected into the center of the first extrudate. Design of the introducing means is such that the second extrudate extends across the flow path of the first. By employing several such introducing means, multiple extrudates can be combined.
The coextrusion of materials through the foregoing apparatus is never streamlined nor, is a balanced flow across the extruded sheet ever achieved. Also, the leading polymer must re-knit i.e., join together without a line of demarcation after splitting to flow around the introducing means. Re-knitting is a major problem whenever polymers of dissimilar viscosity are employed, thereby limiting the usefulness and applicability of the apparatus.
U.S. Pat. No. 3,959,432 discloses a process for the manufacture of composite sheeting having a layer of polystyrene and at least one layer of polycarbonate. The apparatus employed provides for the extrusion of molten polystyrene forming a rod. A minor stream of molten polycarbonate is fed into the polystyrene conduit near the top after which both polymer layers pass through a conventional sheet forming die. Introduction near the top of the conduit is also a provision of U.S. Pat. No. 3,959,431. This system does not provide for the coextrusion of materials having dissimilar melt temperatures and viscosities.
U.S. Pat. No. 3,833,704 provides a similar technique which employs an adaptor to extrude a thin layer of one extrudate upon a portion of the circumference of a rod of a second extrudate. Multiple layers are provided by the addition of other adaptors which can be positioned to extrude onto the rod of second extrudate as well as onto previously extruded layers. Coextrusion of materials having dissimilar melt temperatures and viscosities would not be practical.
U.S. Pat. No. 3,479,425 is somewhat similar to U.S. Pat. No. 3,959,431 in that it provides a slot die, having a rectangular orifice, which introduces an extrudate into a larger flowing body of different extrudate. Multiple slot dies can be employed for additional layers and, if desired, dies having a circular orifice can be substituted forming one or more concentric layers within the larger extrudate body. This method and apparatus suffers many of the problems already stated including non-streamlined flow, lack of balanced flow across the extruded sheet and necessity of re-knitting.
Another known system for producing coextrusions, particularly where multilayer products of two or more materials are desired, provides a plurality of adjacent streams of the materials in alternate and contiguous arrangement. U.S. Pat. No. 3,557,265 is directed toward such a system and employs a manifold having a distribution block which, in its simplist form, divides two streams of material such as a nonexpandable and an expandable resinous material into from about five to 1000 alternate, contiguous layers. The layers emerge from the distribution block and manifold into a transition piece which flattens them to a sheet configuration.
This system requires a very costly manifold and despite its complexity, it does not streamline the flow of materials to the extrusion opening. Lack of streamlining will give rise to stagnation. Thermally sensitive polymers degrade with excess time and temperature both of which occur with stagnation. Moreover, when corrosive polymers degrade, such as those containing chlorine, damage occurs to unprotected metal surfaces such as in the manifold, distributor block and transition piece of the apparatus. Stagnation of non-corrosive polymers gives rise to commercially unacceptable products, inasmuch as the degraded polymer, usually discolored, slowly dissolves into the molten undegraded flow and appears in the product. Intricacy of the passageways in the apparatus also increases the difficulty of cleaning and other maintenance. Another shortcoming of the system is that a costly transition piece must be substituted for extrusion of different sheet widths.
While each of the foregoing patents provides a usable system for coextruding certain polymeric materials, the volummetric amount of one layer with respect to another as well as its juxtaposition is not readily changed. Changeover for different numbers of layers involves substitution of equally costly elements. Also, if a polymer degrades giving off gasses, safe pressures for the system can be exceeded causing damage to the equipment and possibly the operator.